1. Field of the Invention
This invention relates to a conductive silicone rubber composition containing conductive carbon black, and more particularly, to a conductive silicone rubber composition which has a superior storage stability and cures into a satisfactory rubber product through atmospheric hot air vulcanization (HAV).
2. Prior Art
Most rubbery materials are electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10.sup.5 to 10.OMEGA.-cm are conductive rubbers which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its high-temperature resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents like the other rubbery materials.
The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and short fibers surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing these conductive agents, the resistivity of silicone rubber can be reduced to the order of 10.sup.10 to 10.sup.-3 .OMEGA.-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber. Among others, carbon black and metal powders such as silver and nickel are often used where it is desired to produce highly conductive silicone rubber having a resistivity of less than 10.sup.5 .OMEGA.-cm. Carbon black is most often used because of cost.
However, only a limited vulcanization system is applicable to conductive silicone rubber compositions having blended therein acetylene black and other carbon black as a conductive agent when they are continuously molded as by extrusion molding and vulcanized into a length of article such as a seal, gasket and roll. More particularly, typical organic peroxide vulcanization fails to yield satisfactory molded products. If acyl series peroxides commonly used in atmospheric hot air vulcanization (HAV) of conventional silicone rubber compositions, for example, benzoyl peroxide and 2,4-dichlorobenzoyl peroxide are used in carbon black-containing systems, the carbon black acts to restrain vulcanization. If alkyl series peroxides, for example, dicumyl peroxide and di-tert-butyl peroxide are used in carbon black-containing systems, compression molding is permissible, but extrusion atmospheric hot air vulcanization fails to yield satisfactory products because the surface is vulcanized short due to the influence of oxygen in air.
For extrusion atmospheric hot air vulcanization of carbon black-containing silicone rubber compositions, an addition vulcanization approach was conventionally used in order to overcome the above-mentioned difficulty. This approach is to add a platinum series addition reaction catalyst to a mixture of an organopolysiloxane having an alkenyl group and an organohydrogenpolysiloxane having a silicon-attached hydrogen atom capable of addition reaction with the alkenyl group, thus inducing addition reaction curing.
This addition reaction curing approach, however, suffers from the problems of a limited shelf life and poisoning by catalyst poisons such as amines, sulfur and tin. This imposes a serious problem on molding. Typically conductive silicone rubber is used to mold a variety of articles, for example, electromagnetic radiation shielding gaskets, building gaskets, business machine conductive rolls (including charging rolls, transfer rolls, developing rolls, paper feed rolls and fixing rolls), and conductive calendered moldings (such as molded zebra connectors). If conductive silicone rubber is molded by conventional atmospheric hot air vulcanization (HAV), problems arise with respect to moldability and rubber quality. It was thus desired to overcome the above-mentioned problems.
One solution by the inventors is disclosed in Japanese Patent Application Kokai (JP-A) No. 43802/1993 by Shin-Etsu Chemical Co., Ltd. or corresponding USSN 07/812,285 and EP 0493112, which is incorporated herein by reference. Proposed therein is a conductive silicone rubber composition comprising
(A) an organopolysiloxane of the general formula (1): EQU R.sub.a.sup.1 SiO.sub.(4-a)/2 ( 1) PA1 (B) conductive carbon black, and PA1 (C) an organic peroxide of the general formula (2): ##STR1## wherein X is a group of formula (4 ) or (5): wherein m is an integer of 2 to 8, and PA1 (A) an organopolysiloxane of the general formula (1): EQU R.sub.a.sup.1 SiO.sub.(4-a)/2 ( 1) PA1 wherein R.sup.1 groups which may be identical or different are selected from substituted or unsubstituted monovalent hydrocarbon groups and letter a is a positive number of 1.90 to 2.05, PA1 (B) conductive carbon black, PA1 (C) at least one member selected from organic peroxides of the general formulae (2) and (3): ##STR2## wherein X is a group of formula (4) or (5): ##STR3## wherein m is an integer of 2 to 8, R.sup.2 and R.sup.3 are independently a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group--SiR.sub.3.sup.4 wherein R.sup.4 is a methyl, ethyl or phenyl group, and PA1 (D) at least one compound having a group --N.dbd.N--. PA1 (A) an organopolysiloxane of the general formula (1): EQU R.sub.a.sup.1 SiO.sub.(4-a)/2 ( 1) PA1 wherein R.sup.1 is independently selected from substituted or unsubstituted monovalent hydrocarbon groups and letter a is a positive number of 1.90 to 2.05, PA1 (B) conductive carbon black, PA1 (C) an organic peroxide of the general formula (2): ##STR4## wherein X is a group of the following formula (3) or (4): ##STR5## wherein n is an integer of 2 to 8, R.sup.2 and R.sup. 3 each are a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group of the following formula (5): EQU --SiR.sub.3.sup.4 ( 5) PA1 (D) at least one additive selected from the group consisting of a fatty acid of the general formula (6): EQU R.sup.5 (COOH).sub.m ( 6) PA1 wherein R.sup.5 is a hydrogen atom or a saturated or unsaturated, substituted or unsubstituted, monovalent hydrocarbon group having 1 to 30 carbon atoms and m is an integer of 1 to 10, an acid anhydride resulting from molecular condensation of the fatty acid, and an acid anhydride of the general formula (7): ##STR6## wherein R.sup.6 is a saturated or unsaturated, substituted or unsubstituted, polyvalent hydrocarbon group having 1 to 30 carbon atoms and p is an integer of 1 to 3. PA1 (A) an organopolysiloxane of the general formula (1): EQU R.sub.a.sup.1 SiO.sub.(4-a)/2 ( 1) PA1 wherein R.sup.1 is independently selected from substituted or unsubstituted monovalent hydrocarbon radicals, and letter a is a positive number of from 1.90 to 2.05. PA1 (B) an organic peroxide of the general formula (2): ##STR7## wherein X is a radical of the following formula (3), (4) or (5): ##STR8## wherein n is an integer of 2 to 8 and m is an integer of 1 to 4, R.sup.2 and R.sup.3 each are a monovalent hydrocarbon radical having 3 to 10 carbon atoms or a radical of the following formula (6): EQU --SiR.sub.3.sup.4 ( 6) PA1 (C) carbon black. PA1 (A) a compound including a first silicone rubber composition containing a first organopolysiloxane of the average compositional formula (1): EQU R.sub.n.sup.1 SiO.sub.(4-n)/2 ( 1) PA1 wherein R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group, up to 95 mol % of the R.sup.1 groups being methyl groups, and letter n is a positive number of 1.98 to 2.02, and conductive carbon black, and a second silicone rubber composition containing a second organopolysiloxane of the average compositional formula (2): EQU R.sub.m.sup.2 SiO.sub.(4-n)/2 ( 2) PA1 wherein R.sup.2 is a substituted or unsubstituted monovalent hydrocarbon group, at least 98 mol % of the R.sup.2 groups being methyl groups, and letter m is a positive number of 1.98 to 2.02, and PA1 (B) an organic peroxide of the general formula (3): ##STR9## wherein X is a group of formula (4) or (5): ##STR10## wherein a is an integer of 2 to 8, R.sup.3 and R.sup.4 each are independently a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group of formula (6): EQU --SiR.sub.3.sup.5 ( 6)
wherein R.sup.1 groups which may be identical or different are selected from substituted or unsubstituted monovalent hydrocarbon groups and letter a is a positive number of 1.90 to 2.05,
R.sup.2 and R.sup.3 are independently a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group --SiR.sub.3.sup.4 wherein R.sup.4 is a methyl, ethyl or phenyl group.
This conductive silicone rubber composition can be extrusion or calender molded and satisfactorily vulcanized by atmospheric hot air vulcanization, that is, effectively molded and vulcanized into silicone rubber having good physical properties and conductivity.
Containing research works, we found that the previously proposed conductive silicone rubber composition exhibited satisfactory physical properties and workability immediately after its preparation, but experienced an increase of plasticity and scorching phenomenon with the lapse of time. There is a need for increasing the shelf stability of such a composition.